1. Field of the Invention
The present invention relates to a system for optically transmitting a frequency-division multiplex signal occupying a broad frequency band which is necessary in cable television systems where a plurality of signals each containing a television or radio program have to be transmitted.
2. Description of the Prior Art
A system of the kind, above-mentioned in which the frequency-division multiplex signal is composed of four television signals and several radio, telephone, and data signals, is known from "IEEE Transactions on Communications", Vol. COM-29, No. 6, June 1981, pp. 868 to 885 (FIG. 13 on p. 878). It occupies a frequency band extending from 54 to 108 MHz. The frequency-division multiplex signal is formed by frequency-modulating different carriers which are chosen to lie within a band only one octave wide.
For the number of television and radio programs to be transmitted in present-day cable television systems (at least 35 television programs and 30 VHF radio programs), this prior art system is not sufficient.
In the prior Patent Application DE-OS 39 07 495, it is proposed to use the frequency-division multiplex signal provided by the KTV head station (KTV=German abbreviation for cable television), which occupies a frequency band from 40 to 440 MHz, directly, i.e., without any conversion, to intensity-modulate the semiconductor laser.
It has turned out that second-, third-, and higher-order intermodulation products are caused by such modulation, with the second-order products dominating. Therefore, the semiconductor laser may only be driven with a small signal. As a result, the system tolerates an optical link loss of only few dB.